Method for recovering thorium and rare earth elements from rare earth waste residues

Abstract

A method for recovering thorium and rare earth elements from rare earth waste residues includes the steps of (1) mixing rare earth waste residues with an inorganic acid and heating to obtain a stock solution containing thorium and rare earth elements; (2) extracting thorium and rare earth elements from the stock solution with an organic phase containing an extractant; (3) washing the organic phase obtained after extraction in step (2) with a washing solution to move rare earth elements into the aqueous phase and leave thorium in the organic phase; (4) back-extracting the organic phase containing thorium obtained in step (3) with a back-extraction solution to extract move thorium in the organic phase into the aqueous phase. The extractant contains alkyl phosphonic acid monoalkyl ester and dialkylphosphinic acid.

The invention claimed is: 
     
       1. A method for recovering of thorium and rare earth elements from rare earth waste residues, comprising:
 (1) mixing rare earth waste residues with an inorganic acid and heating to obtain a stock solution containing thorium and rare earth elements; 
 (2) extracting thorium and rare earth elements from the stock solution with an organic phase containing an extractant; 
 (3) washing the organic phase obtained from step (2) with a washing solution to move rare earth elements into an aqueous phase, wherein thorium remains in the organic phase; 
 (4) back-extracting the organic phase containing thorium obtained from step (3) with a back-extraction solution to move thorium from the organic phase into the aqueous phase, wherein the extractant is a mixture of one or more alkyl phosphonic acid monoalkyl esters of formula (I) and one or more dialkylphosphinic acids of formula (II): 
 
       
         
           
           
               
               
           
         
         wherein R 1 , R 2 , R 3 , and R 4  are same or different alkyl groups. 
       
     
     
       2. The method of  claim 1 , wherein, in formula (I), R 1  and R 2  are the same or different, and are each independently a C 3 -C 20  linear alkyl. 
     
     
       3. The method of  claim 1 , wherein the organic phase in step (2) further contains a phase modifier and a diluent. 
     
     
       4. The method of  claim 3 , wherein, in the organic phase in step (2):
 the extractant is a mixture of 2,4,4-trimethylpentyl phosphonic acid 2,4,4-trimethylpentyl ester and bis(2,4,4-trimethylpentyl) phosphinic acid, the phase modifier is iso-octyl alcohol, the diluent is sulfonated kerosene, and the volume percentages of the extractant, the iso-octyl alcohol, and the sulfonated kerosene are 10 vol %, 20 vol %, and 70 vol %, respectively; 
 the extractant is a mixture of 2,2,4-trimethylpentyl phosphonic acid 2,2,4-trimethylpentyl ester and bis(2,2,4-trimethylpentyl) phosphinic acid, the phase modifier is tributyl phosphate (TBP), the diluent is xylene, and the volume percentages of the extractant, TBP, and xylene are 20 vol %, 20 vol %, and 60 vol %, respectively; 
 or 
 the extractant is a mixture of 2,3,4-trimethylpentyl phosphonic acid 2,3,4-trimethylpentyl ester and bis(2,3,4-trimethylpentyl) phosphinic acid, the phase modifier is sec-octyl alcohol and the volume percentages of the extractant, the sec-octyl alcohol, and the diluent are 10 vol %, 20 vol %, and 70 vol %, respectively. 
 
     
     
       5. The method of  claim 3 , wherein, in the organic phase, the volume percentages of the extractant, the phase modifier, and the diluent are 1-40 vol %, 5-30 vol %, and 55-94 vol %, respectively. 
     
     
       6. The method of  claim 3 , wherein, the phase modifier is one or more selected from the group consisting of n-octanol, iso-octyl alcohol, sec-octyl alcohol, tributyl phosphate (TBP), and dimethylheptyl methylphosphate. 
     
     
       7. The method of  claim 3 , wherein the diluent is one or more selected from the group consisting of alkanes and aromatic hydrocarbons. 
     
     
       8. The method of  claim 1 , wherein the concentration of the inorganic acid in step (1) is 10-18 mol/L, and the inorganic acid is one or more selected from the group consisting of hydrochloric acid, nitric acid, and sulfuric acid. 
     
     
       9. The method of  claim 1 , wherein, in step (1), the rare earth waste residues are mixed the with inorganic acid, and then heated to 60-150° C. and leached for 0.5-3 hours. 
     
     
       10. The method of  claim 1 , wherein, in step (1):
 the stock solution is obtained by mixing the rare earth waste residues with hydrochloric acid, and then heating, leaching, and diluting, wherein, in the stock solution, the total concentration of rare earth elements is 0.35 mol/L, the concentration of free hydrochloric acid is 0.30 mol/L, and the molar ratio of thorium to total rare earth elements is 1:10; 
 the stock solution is obtained by mixing the rare earth waste residues with sulfuric acid, then heating, leaching, and diluting, wherein, in the stock solution, the total concentration of rare earth elements is 0.15 mol/L, the concentration of free sulfuric acid is 0.50 mol/L, and the molar ratio of thorium to total rare earth elements is 1:2.6; 
 or 
 the stock solution is obtained by mixing the rare earth waste residues with nitric acid, then heating, leaching, and diluting, wherein, in the stock solution, the total concentration of rare earth elements is 0.25 mol/L, the concentration of free nitric acid is 0.40 mol/L, and the molar ratio of thorium to total rare earth elements is 1:4. 
 
     
     
       11. The method of  claim 1 , wherein, in step (3), the washing solution is hydrochloric acid, nitric acid, or sulfuric acid at a concentration of 0.1-1.0 mol/L. 
     
     
       12. The method of  claim 1 , wherein, in step (4), the back-extraction solution is hydrochloric acid, nitric acid, or sulfuric acid at a concentration of 2.0-6.0 mol/L. 
     
     
       13. The method of  claim 1 , wherein, in step (2), a volume ratio of the organic phase to the stock solution is 0.1:1 to 2:1. 
     
     
       14. The method of  claim 1 , wherein the extraction process is carried out in a plurality of mixer settlers, containing n-stage extraction, m-stage washing, and j-stage back-extraction, wherein n=4-6, m=5-8, and j=4-6. 
     
     
       15. The method of  claim 1 , wherein, in formula (I), R 1  and R 2  are the same or different, and are each independently a C 6 -C 12  branched alkyl. 
     
     
       16. The method of  claim 1 , wherein, in formula (II), R 3  and R 4  are the same or different, and are each independently selected from the group consisting of C 3 -C 20  linear and branched alkyls. 
     
     
       17. The method of  claim 1 , wherein, in the extractant, the volume percentages of alkyl phosphonic acid monoalkyl ester and dialkylphosphinic acid are 30-90 vol % and 10-70 vol %, respectively. 
     
     
       18. The method of  claim 1 , wherein, a ratio of the mass in kilograms of the rare earth waste residues to the volume in liters of the inorganic acid is 1:1-8. 
     
     
       19. The method of  claim 1 , wherein, in the stock solution, the total concentration of rare earth elements is 0.1-0.5 mol/L. 
     
     
       20. The method of  claim 1 , wherein the washing solution is 0.5 mol/L hydrochloric acid, 0.3 mol/L sulfuric acid, or 0.5 mol/L nitric acid.